Oxidant Production, oxLDL Uptake, and CD36 Levels in Human Monocyte–Derived Macrophages Are Downregulated by the Macrophage-Generated Antioxidant 7,8-Dihydroneopterin

Abstract

The severity of atheroma burden in patients strongly correlates to increasing levels of plasma neopterin, the oxidation product of 7,8-dihydroneopterin. Interferon-γ stimulation of macrophages causes the synthesis of 7,8-dihydroneopterin, a potent antioxidant that inhibits oxidative damage to cells, and the cytotoxicity of oxidized low-density lipoprotein (oxLDL) to monocyte-like U937 cells but not THP-1 cells. With human monocyte-derived macrophages (HMDMs), oxLDL triggered a large oxidative stress, causing the rapid loss of cellular glutathione, glyceradehyde-3-phosphate dehydrogenase (GAPDH) inhibition, and eventual loss of viability without caspase-3 activation. Inhibition of oxLDL cytotoxicity to HMDMs occurred at 7,8-dihydroneopterin concentrations >100 μM. The oxLDL-mediated glutathione loss and GAPDH inactivation was inhibited by 7,8-dihydroneopterin. 7,8-Dihydroneopterin rapidly entered the HMDMs, suggesting that much of the protective effect was scavenging of intracellular oxidants generated in response to oxLDL. OxLDL uptake by HMDMs was reduced by 30% by 7,8-dihydroneopterin. Immunoblot analysis suggests that this decrease in oxLDL uptake was due to a significant downregulation in the levels of CD36. These results imply that 7,8-dihydroneopterin protects human macrophages both by scavenging oxidants generated in response to oxLDL and by decreasing CD36-mediated uptake of oxLDL.

The arterial microenvironment: the where and why of atherosclerosis

“…Necrotic core progression is likely due to the formation of cholesterol crystals in the cytosols leading to inflammasome activation and subsequent IL-1α/β production [32]. In addition M1 macrophages can contribute to oxidative stress observed in the plaque via the production of hydrogen peroxide, superoxides, neopterin (the oxidized product of 7, 8-dihydroneopterin) and myeloperoxidases [33, 34]. These radicals can promote further oxidation of LDLs within the plaques, as well as the initiate of endoplasmic reticulum stress that can initiate apoptosis.…”

Neopterin and 7,8-dihydroneopterin are generated within atherosclerotic plaques

“…We have previously shown that 2 μmol/L of 7,8-dihydroneopterin inhibits LDL oxidation [13], although 50-100 μmol/L is required to prevent oxLDL-or HOCl-induced cell death [16][17][18]. It is possible that macrophage cell death occurs at a very localised cellular level, so concentrations of 7,8-dihydroneopterin around the cells may reach protective levels.…”

“…Neopterin causes significant cellular stress at elevated concentrations [10][11][12], while 7,8-dihydroneopterin has been shown to be a potent antioxidant capable of inhibiting copper, peroxyl radical and cell-mediated oxidisation of low-density lipoprotein (LDL) [13][14][15]. Macrophage uptake of oxidised LDL (oxLDL) is decreased by 7,8-dihydroneopterin-mediated down-regulation of the CD36 scavenger receptor [16]. Macrophage necrosis by HOCl, oxLDL and peroxyl radicals is also inhibited by 7,8-dihydroneopterin [16][17][18][19].…”

“…Macrophage uptake of oxidised LDL (oxLDL) is decreased by 7,8-dihydroneopterin-mediated down-regulation of the CD36 scavenger receptor [16]. Macrophage necrosis by HOCl, oxLDL and peroxyl radicals is also inhibited by 7,8-dihydroneopterin [16][17][18][19]. HOCl reacts extremely rapidly with 7,8-dihydroneopterin to generate neopterin [20,21], suggesting that a significant amount of the neopterin measured in serum and urine may be from the scavenging of HOCl released by neutrophils and macrophages during inflammation.…”

Lysosomes ‘in control’

“…To manage this re-equilibrium of oxidation, pro-oxidant mechanisms can be reduced and/or antioxidant mechanisms can be increased, with the latter being the most applied approach. In relation to its effect on oxLDL, antioxidants have been shown to decrease oxLDL levels after administration of the antioxidant cumin 175 and the macrophage-generated antioxidant 7,8-dihydroneopterin, 176 proving antioxidants as a proper approach to decrease oxLDL.…”